1. Technical Field
The present invention relates to an electrophoretic display sheet, an electrophoretic display apparatus and an electronic device, and more specially relates to an electrophoretic display sheet, an electrophoretic display apparatus provided with the electrophoretic display sheet and an electronic device provided with the electrophoretic display apparatus.
2. Related Art
As disclosed in JP-A-2008-58358, there is known an electrophoretic display utilizing electrophoretic particles as an image display section of an electronic paper. The electrophoretic display has superior portability and thrifty power consumption. Therefore, the electrophoretic display is suitably used as the image display section of the electronic paper.
A display device (microcapsule-type electrophoretic display panel) is disclosed in JP-A-2008-58358. The display device includes a front electric plate, a back electric plate disposed opposite the front electric plate, and a capsule-type electrophoretic layer provided therebetween. In the capsule-type electrophoretic layer, a number of microcapsules in which a dispersion liquid is filled are included. In the dispersion liquid, positively-charged white particles and negatively-charged black particles (colored particles) are dispersed.
In such a display device, when a voltage is applied to between the front electric plate and the back electric plate, the white particles and the black particles dispersed in the microcapsules are moved to a predetermined direction, respectively. This makes it possible to change a color (display color) visibly observed through the front electric plate.
Concretely, the voltage is applied to between the front electric plate and the back electric plate so that the front electric plate is charged negatively and the back electric plate is charged positively. By doing so, the white particles are moved to the front electric plate and the black particles are moved to the back electric plate. As a result, the image color is changed to a white color.
Conversely, the voltage is applied to between the front electric plate and the back electric plate so that the front electric plate is charged positively and the back electric plate is charged negatively. By doing so, the white particles are moved to the back electric plate and the black particles are moved to the front electric plate. As a result, the image color is changed to a black color.
As described above, the display device utilizes the electrophoretic particles as disclosed in JP-A-2008-58358. In such a display device, it is required that the black particles are moved from the back electric plate to the front electric plate in order to change the display color from the white color to the black color. Therefore, it is required for the black particles to be moved in a long moving distance. A moving distance of the white particles in changing the display color from the black color to the white color is also the same as that of the black particles.
Therefore, in such a display device, when the display color is switched (changed) from the white color to the black color, a long period of time is required. In other words, there is a problem (first problem) in that responsiveness (reactivity) is poor when the display color is changed at a low.
Further, when the display color is changed to the white or black color, the white and black particles are moved to an opposite direction to each other in the microcapsules. Therefore, in the case where the display color is changed from the black color to the white color, the black particles are surrounded among a large number of white particles at the time of passing the white particles. As a result, there is a case that brightness of the white color is reduced. Therefore, there is another problem (second problem) in that display contrast is reduced and thus visibility of the display is made poor.
In this regard, in order to solve the first problem described above, it may be conceived to make a charge level of the white and black particles higher, thereby making mobility of these particles larger. However, if the charge level of the white and black particles is too high, the white and black particles are electrostatically-adsorbed to each other, thereby producing particle aggregates constituted of the white and black particles. Due to this particle aggregate, a problem is caused in that display contrast is reduced.
For these reasons, in the above display device disclosed in JP-A-2008-58358, it is difficult to satisfy both superior responsiveness and superior color display performance with high contrast in changing the display color.